The technology isn't incredibly immature. On the contrary, it's understood to the tiniest detail. If it doesn't work, it won't work.
Maybe the efficiency can be improved upon with a small-scale 'gravity battery', but water has massive advantages when it comes to scaling it up, as well as being able to combine it with traditional hydro, where nature moves much of the water for you.
I've often wondered what scale of a system would be required to power a fully off the grid home at say 30kwh/ day, for say, 6 days? I'm imagining a tank based system with two tanks where the fluid is effectively just exchanged (so one doesn't have to account for the environmental impacts of a dam or for losses due to evaporation)..
After some googling: https://www.edie.net/library/Using-pumps-as-turbines-to-gene...